Electron discharge devices



Feb. 10, 1959 H. D. DOOLITTLE 2,373,401

, ELECTRON DISCHARGE DEVICES Filed Dec. 12, 1956 :s Sheets-Sheet 1INVENTOR. HOWARD o DOOLITTLE AGENT Feb. 10, 1959 -H. D. DOQLITTLEELECTRON DISCHARGE DEVICES Filed Dec. 12. 1956 5 Sheets-Sheet I5 FIG]INVENTOR.

HOWARD o. DOOLITTLE AGENT United States Patent ELECTRON DISCHARGEDEVICES Howard D. Doolittle, Stamford, Conn assignor to MachlettLaboratories, Incorporated, Springdale, Conn., a corporation ofConnecticut Application December'll, 1956, Serial No. 627,834

6 Claims. (Cl. 313-498) tures which do little to provide or maintain ahigh plate current to grid current ratio or used complicated beamingstructures which involved parts difficult to manufacture and assemble,and in some cases prior art tubes were not sufiiciently rugged towithstand transiental faults such as. flash arcs in the tube which aredifiicult to eliminate completely. Such flash arcs are sometimes causedby transients in the power mains or byparasiticoscillations of the tubescircuits.

The present invention is directed primarily to the provision of anelectron tube wherein a selected normal or high amplification factor, i.e. approximately from 10 to 100, is achieved by positioning therespective pairs of parallel grid and filament wires of substantiallycylindrical electrodes in radial alignment with respect to the axis ofthe electrode structure so that each filament wire will be in the mosteffective portionof the field influenced by a respective grid wire.

An additional feature is the provision of means for reducing gridcurrent in such a structure either by restricting cathode emission tothose portions of the filament wires which lie farthest from the gridwires, or by providing ashield grid between the filament and controlgrid, which shield grid is maintained at or near cathode potential forbeaming the electrons emitted by the cathode so that they will notimpinge upon the control grid wires. Either of the methods will reducegrid current and obviate the necessity for cooling the control grid, andalso at the same time will provide a large safety factor under transientfaults.

A further control of the amplification factor so as to provide a tubewith a relatively high mu is achieved, in accordance with thisinvention, by controlling the spacing of the respective wires of thegrid structure and controlling their diameters relative to the diametersof the cathode wires so as to produce the desired mu on the usefulelectron emitting areas of the cathode.

' Known means andmethods of increasing the amplification factor of anelectron tube are by moving the grid away from the anode or bydecreasing the space between grid wires, or a combination of both.However,

I further modification of the invention.-

the results of such means and methods have not been entirelysatisfactory, particularly in a structure wherein the grid wires wereclosely spaced, because the problems of grid heating and ratio of platecurrent to grid current became serious due to lack of means forpreventing the grid from intercepting current. e

2,873,401 Patented Feb. 10, 1959 "ice It is known also that grid currentin a tube can be reduced by providing a grid with relatively tine wireswhich, due to their small diameters, will not intercept as manyelectrons as relatively large diameter wires. This results in a lowerscreening fraction for a given grid wire pitch, which may besatisfactory for certain. applications. However, in most power tubes theuse of fine wires is not desirable because of their fragility.

In order to minimize grid driving power, mu should be kept as high aspossible and intercepted grid current should be kept as low as possible.This goal is achieved in certain known tube structures but suchstructures require a relatively large number ofvery accurately alignedparts, some of the parts having shapes requiring costly fabrication.

It is, therefore, a primary object of this invention to provide asimple, rugged, and relatively inexpensive electron tube having aselected normal or relatively high amplification factor with low drivingpower and capable of offering high power levels.

Another object is to provide an electron tube with a novel electrodestructure whereby a normal or relatively high amplification factor isobtained without undesirable rise in grid current and resultanttemperatures, thus eliminating any requirement for grid cooling.

A further object is the provision of an electrode structure within anelectron tube wherein cathode emission is restricted to controlled areasthereof as determined by the relative spacings of grid wires, wherebythe tube is pro- 'vided with a controlled normal or high amplificationfactor. A still further object is the provision in a tube of the abovecharacter of means at or near cathode potential for beaming the electronHow to provide a still higher mu and substantially reducing electronbombardment of the control grid with resultant reduction in incidentalgrid current and heating.

Other objects and advantages of the invention will become apparent fromthe following description taken in connection with the accompanyingdrawings, in which,

Fig. l is an axial sectional view of an electron tube embodying theinvention;

Fig; 2 is a reduced sectional view taken on line 2-2 of Fig. 1;

Fig. 3 is a schematic diagram illustrating diagrammatically the electronemission of a filament having restricted emission areas; I

Fig. 4 is a view similar to Fig. 1 illustrating a modification of theinvention; n

Fig. 5 is a sectional view taken on line 55 of Fig. 4;

Figs. 6 and'7 are diagrams illustrating certain principles of theinvention;

Fig. 8 is a schematic diagram illustrating diagrammatically the beamingeffect produced in the tube shown in Fig. 4; and

Fig. 9 is a sectional view similar to Fig.-

showing a k Referring to' the drawings, the electron tube shown in Fig.1 includes an anode 10 formed as a deep cylindrical cup of highlyconductive metal such as copper. Mounted around the anode 10 near theopen end thereof is a ring 11 which provides the anode terminal. A shorttubular member 12, preferably of Kovar, is sealed throughout one end tothe terminal ring 11 and has its other end sealed to one end of adielectric ring 13. The opposite end of ring 13 is likewise sealed to asecond tubular Kovar member 14 which is carried by an annular gridterminal 15.

Grid terminal 15 also carries a tubular grid support 16 which extendsinwardly of the structure toward the anode 10, the grid support 16having a supporting ring 17 screwed .05 otherwise secured in coaxialrelation to its inner end. The ends of a plurality of control grid wires13 are secured to the inner surface of ring 17 and extend into theinterior of the anode to form a cagelike structure of parallel wires,which structure, in cross-section, defines a circle. The ends of thecontrol grid Wires 18 within the anode are secured to a disc 19 whichmaintains the wires in desired spaced relation.

A second dielectric ring 20, of smaller diameter than ring 13, is sealedat one end by a Kovar member 21 to the grid terminal 15, and at itsother end by a Kovar member 22 to a cathode terminal ring 23. A firstcupshaped cathode support 24 extends inwardly from terminal ring 23. Oneend of a plurality of filament supporting posts 25 are mounted in theend 24a. of the support 24 and extend into the control grid structure. Asecond group of posts 26 are arranged alternately with posts 25 so as todefine therewith, in, cross-section, a circle of posts within the gridwires 18. Posts 26 extend. through clearance openings 27 in the end 24aof the cup-shaped support 24 and their ends are mounted in the end 28aof a second cup-shaped support 28 which is located within and. spacedfrom support 24. The, open end of support 28 is sealed to a secondcathode terminal ring 29 which is sealed, by a pair of Kovar rings 30and 31 having a vitreous ring 32 therebetween, to the terminal ring 23.

The inner ends of the filament support posts 25 and 26 extend slightlyinto the control grid structure and each carries a respective filamentwire 33. The wires 33 are mounted on the. posts. 25-26 by means of clipsor clamps 34 and extend substantially parallel with grid wires 18. Incross-section, wires 33 define a circle of wires. within and inpredetermined spaced relation to the circle of control grid wires. Theinner ends of wires 33 are joined at 35 to complete the cathodestructure. An exhaust tubulation 36 is sealed in the open end of support28.

In the manufacture of electron tubes of. the triode type, it isnecessary to accurately position the grid wires with respect to both theanode and the cathode. Various spacings of the electrodes relative toone another may be used to provide tubes having diiferent amplificationfactors. A common arrangement is to dispose the filament wires of thecathode opposite the spaces between the grid wires so that electronsemitted by the cathode may pass through the spaces directly to theanode. In such cases, where the respective wires in the grid are spacedapart a distance which is substantially the same as the grid-cathodespacing, the mu across, thecathode is substantially constant. Toincrease the amplification of such a tube, common practice has beeneither to move the anode away from the grid or to decrease the spacingsbetween the respective grid wires, or both. Likewise, it has been commonprocedure to decrease the amplification factor of a. tube either bydecreasing the space between grid and. anode or by increasing thespacings between grid wires, or both. I

I have found, however, that the amplification factor of a tube can beincreased by positioning respective pairs of grid and filament wiresinradial alignment with respect to the axis of the electrode structure.The grid wires 18 thus are each located in the shortest path between arespective filament wire 33 and the nearest portion of the anode 10 asshown in Fig. 2. By thus being directly in the electron paths betweenthe filament wires and anode, the grid wires have their greatestinfluence on the cathode emission.

In a structure as described, upon operation of the tube the filamentwires 33 will emit a copious supply of elec! trons which are intended toflow to the anode 10. It is apparent, however, that a large number ofthe electrons will travel directly to the grid wires 18 due to the particular positions of the grid Wires directly in the electron paths. Suchbombardment of the grid wires causes undesired grid current.

In accordance with another feature of this invention, bombardment of thegrid wires is considerably reduced by providing the filament wires withlimited emission areas. For example, the areas of the filament wiresnearest the grid wires can be made substantially non-emissive while theremaining areas can be made to emit copiously. Since electronstheoretically leave the surfaces of the filament wires substantiallynormal to the tangent of the surface at the point of emission, suchemission will thus pass by the grid wires. This is exaggeratedlyillustrated in Fig. 3 wherein a filament wire. 40 is made to be emissiveonly at the surface areas indicated by the double line 41. The electronstherefrom will fiow in initially generally arcuate paths, indicated bydotted lines, through the spaces between the grid Wires and toward theanode.

- Thus, a. grid wire 52 will not be bombarded because of the fact thatthe filament wire 4t) will not emit, electrons from surface 43 which isnearest grid Wire. 42.

The filament wires can be provided with limited emis; sion areas byconventional methods which are slightly modified fromthe usual practice.If the filament wires 40 are formed of thoriated tungsten, for example,they can be provided with limited emission areas by masking the surfaces43 during the carburizing process so that only the unmasked surfaceswill be carburized and thereby made capable of copious electronemission. Another method which may be used is to coat only the surfaceto be carburized with aquadag; before the carburizing step. Aftercarburizing, surface 43. will be, substantially nonemissive compared tothe remaining surfaces of the Wires. If the filament wires areoxide-coated, wires, with the emission being provided by the oxidecoating, the coating can be applied only to the surfaces of the wireswhich are to be emissive, or the entire surface of each, wire can beoxide coated, with the non-emissive areas being subsequently masked by aseparate thin overlying coating of non-emissive material.

Other methods of making filament wires only partially emissive can beaccomplished so that the wires can be used in connection with thepresent invention.

The amplification factor of a tube can be increased also by increasingthe spacings between grid wires to an extent where the spacings begin toget larger than the grid-to-cathode distance. This causes the efiectivemu tobecome variable across the face of a planar or cylindrical cathode.Under these circumstances the parts of the cathode situated centrallybetween the grid wires have a much lower mu than those parts immediatelyadjacent the grid wires.

This is illustrated in Figs. 6 and 7 wherein a cathode 44 is spaced aknown distance a from grid wires 45 and the spacing between grid wiresis indicated by b. It will be noted that b is larger than a, Thisproduces a variable mu across the face of the cathode which isllustrated. by curve 0 in Fig, 7. The high peaks in curve 0 indicate therelatively high mu adjacent each grid wire 45,, nd the. dip in the curveindicates th t h m is lower between grid wires.

Each of; the. pea s in curve. C is r lativelylevel or constantthroughout a restricted length as indicated at d. While the mu in areasd is not absolutely constant, the variation is relatively small and isnot sufficient to cause distortion of the waveshape imposed on the tube.Therefore, in accordance with this invention, I have provided a tube asshown in Fig. 4 wherein the parts common to the tube. shown in Fig. 1have been given like characters of reference with primes added. In thismodification of the invention I have spaced the grid wires 18' apart adistance b which is larger than the grid cathode spacing a, whichproduces a variable mu across the cylinder defined by the cathode, andhave located the filament wires 33 of the cathode at the locations wherethe mu is highest. Thus the filament wires 33 and respective grid wires18 are disposed in pairs extending radially from the center of thecathode structure, as seen in Fig. 5. Since the filament wires extendparallel with the grid wires, there is no emissive portion of thecathode in the relatively low mu areas between the grid wires, allemission being restricted to the high mu areas.

It is apparent that the actual mu value can be ad justed as desired byappropriately spacing the grid wires from each other and/or by adjustingthe space between the grid and anode, so as to produce a curve whereinthe relatively level areas a! are located at the desired mu. Inaccordance with this invention, however, the cathode emission isrestricted to those portions of the cathode which are within thoseareas.

In the example shown in Figs. 6 and 7, a mu of about 96 is obtainable.However, if a higher or lower mu is desired it is merely necessary, inaccordance with this invention, to appropriately space the wires 45 soas to produce a curve wherein the relatively level areas d are locatedat the desired mu, and the cathode emission is restricted to thoseportions of the cathode which are within those areas.

Another method is shown in the structure of Figs. 4, and 8 forsubstantially reducing the cathode emission which impinges upon the gridwires. Such means is in the form of a shield grid similar to the controlgrid but of smaller diameter. The shield grid is located between thecontrol grid and filament and is comprised of a plurality 'of wires 38,each of which wires extends parallel with and between respective controlgrid and filament wires 18 and 33, the shield grid wires 38 beingsecured at one end to a disc 39 which maintains them in desired spacedrelation.

The opposite ends of shield grid wires 38 are, in accordance with thisinvention, connected to a source of potential which will repelelectrons. For example, the wires 38 may be mounted on the filamentsupport 24 if desired, as shown in Fig. 4, and in this case the shieldgrid will be at ground or cathode potential. If desired, and at anincrease in the resultant cost of a tube, an additional terminal may beprovided in the tube envelope, with the shield grid being suitablyconnected to the added terminal. In such a device the terminal andshield grid may be provided with a negative potential which is the sameas or somewhat negative with respect to cathode potential.

By providing a shield grid of this type, the electron supply emanatingfrom the filament wires 33 will be beamed by the shield grid wires 38,as indicated diagrammatically in Fig. 8, and will pass on to the anodewith substantially no bombardment of the control grid wires 18'. Thisprovides little or no grid current and consequently requires no coolingmeans for dissipating excess heat from the control grid.

This last-described method of preventing grid current is equallyadaptable to the tube structure shown in Fig. 1.

It is also possible to apply the present invention to tubes wherein thecathode is cylindrical in shape. This is shown in Fig. 9 wherein acylindrical cathode 46 is positioned coaxially within an anode 4'7 andwithin a control grid structure 48 and a shield grid structure 49.

In this structure the distance between the wires of the control grid 48is considerably larger than the control grid to cathode spacing, whichproduces the variable mu across the face of the cathode 46 as desired inaccordance with the invention.

The cathode 46 is provided on its outer surface with elongated strips5t) of electron emitting material, such as an oxide coating. Strips 50extend parallel with the grid wires 48 and 49 and are of controlledwidth so that no portions thereof extend transversely out of the high muareas, the emission thus being restricted to the high mu areas.

In providing electron tubes of the high mu type constructed' asdescribed, the structures employ the so-called beaming efiect to reducecurrents intercepted by the control grid. This low interception not onlyreduces heating of the control grid wires, and thus aids in re ducingdeformation of the control grid due to uneven heating, but also reducesdistortion of the input waveshape.

From the foregoing description it will be apparent that I have providedan electron tube having improved characteristics in accordance with theobjects of the invention. While the invention is primarily concernedwith power transmitting tubes, it will be obvious to those skilled inthe art that the structures and tube designs given herein are applicableto negative grid operation.

While the novel features of the invention have been shown and describedand are pointed out in the annexed claims, it is to be understood thatvarious omissions, substitutions and changes may be made by thoseskilled in the art without departing from the spirit of the invention.Therefore, it is to be understood that all matter shown or described isto be interpreted as illustrative and not in a limiting sense.

I claim:

1. An electron tube having a high amplification factor comprising anenvelope containing an anode, a cathode and a control grid, the controlgrid having a plurality of parallel wires positioned substantiallyparallel with and at predetermined distances from and between the anodeand cathode and spaced apart a distance which is greater than thegrid-to-cathode spacing whereby a variable mu is produced across thecathode, which mu varies from relatively high levels adjacent each gridwire to relatively low levels in areas adjacent the spaces between thegrid wires, the cathode having a plurality of spaced parallel electronemitting portions positioned in the areas where the mu across thecathode is relatively high, and the grid wires each being located in'theshortest path between a respective cathode emitting portion and thenearest surface of the anode for controlling the flow of electrons fromthe cathode to the anode.

2. An electron tube having a high amplification factor comprising anenvelope containing an anode, a cathode, and a control grid, the controlgrid having a plurality of parallel wires positioned substantiallyparallel with and at predetermined distances from and between the anodeand cathode and spaced apart a distance which is greater than thegrid-to-cathode spacing whereby a variable mu is produced across thecathode, which mu varies from relatively high levels adjacent each gridwire to relatively low levels in areas adjacent the spaces between thegrid electron emitting portions, the cathode having a plurality ofspaced parallel electron emitting portions positioned in the areas wherethe mu across the cathode is relatively high, and thegrid wires eachbeing located in the shortest path between a respective cathode emittingportion and the nearest portion of the anode for controlling the flow ofelectrons from the cathode to the anode, and means for preventing themajor portion of the electrons emitted by the cathode from impingingupon the grid wires, whereby grid current is reduced.

3. An electron tube having a high amplification factor comprising anenvelope containing a hollow anode, a cathode embodying a plurality ofspaced electron emitting wires defining a cylinder extendinglongitudinally of and parallel within the anode, and a control gridlocated between the anode and cathode and embodying a plurality of gridwires extending substantially parallel with the cathode and positionedwith respect to one another so as to define acylinder encircling thecathode in predetermined spaced relation therewith, the grid wires beingspaced apart a distance which is greater than the grid-to-cathodespacing whereby a variable mu is produced across the cylinder defined bythe wires of the cathode, which variable mu varies from. relatively highlevels adjacent each grid wire to relatively low levels in the areaadjacent the spaces between thegridrwires, the electron emitting wireseach being positioned in the areas where the mu is relatively high andthe grid wires each being located in the shortest path between arespective electron emitting wire and the nearest portion of the anode.

4. An electron tube having a high amplification factor comprising anenvelope containing, a hollow anode, a cathode embodying aplurality ofspaced electron emitting wires defining a cylinder, extendinglongitudinally of and parallel within the anode and cathode andembodying a plurality of grid wires extending substantially parallelwith the cathode and. positioned with respect to one another so as .todefine a cylinder encircling the cathode in predetermined spacedrelation therewith, the grid wires being spaced apart a distance whichis greater than the grid-to-cathode spacing whereby a variable mu isproduced across the cylinder defined by the wires of the cathode, whichvariable mu variesv from relatively high levels adjacent each grid wireto relatively low levels in the area adjacent the spaces between thegrid wires, the electron emitting wires each being positioned in theareas where the mu is relatively high and the grid wires each beinglocated in the shortest path between a respective electron emitting wireand the nearest portion of the anode, and means for preventing the majorportion of the electrons emitted by the cathode from impinging upon thegrid wires, whereby grid current is reduced.

5. An electron discharge device as set forth in claim 2 wherein thecontrol grid wires and the electron emitting portions of the cathode allextend longitudinally of the cathode.

6, An electron tube having. a high amplification factor comprising anenvelope containing an anode, a cathode, and a control electrode havinga plurality of parallel effective portions positioned between and atpredetermined distances from the anode and cathode, said efiectiveportions being spaced apart a distance which is greater than thecathode-to-control electrode spacing whereby a variable Inn is producedacross the cathode, which mu varies from relatively high levels adjacenteach effective portion of the control electrode to relatively low levelsin areas adjacent the spaces between said effective portions, thecathode having a plurality of spaced parallel electron emitting portionspositioned in the areas where the mu across the cathode is relativelyhigh, and the elfective portions of the control electrode each beinglocated in the shortest path between a respective cathode emittingportion and the nearest surface of the anode for controlling the flow ofelectrons from the cathode to the anode.

References Cited in the file of this patent UNITED STATES PATENTS2,067,529 Heising Jan. 12, 1937 2,090,006 Knoll et al. Aug. 17, 19372,459,792 Chevigny Jan. 25, 1949 2,719,244 Dailey Sept. 27, 19552,727,177 Dailey Dec. 13, 1955

